1. Field of the Invention
The present invention relates to multi-carrier CDMA (Code Division Multiple Access) radio transmitting method and apparatus, and channel estimation method and apparatus for a multi-carrier CDMA radio transmitting system such as a multi-carrier CDMA radio transmitting system, an orthogonal-frequency-division-multiplexing (OFDM) radio transmitting system or the like, and, in particular, to multi-carrier CDMA radio transmitting method and apparatus enabling transmission of information at various transmission rates, and channel estimation method and apparatus for a multi-carrier CDMA radio transmitting system by which a channel estimation value of each sub-carrier is adaptively controlled according to a state of the radio channel.
2. Description of the Related Art
Currently, a digital mobile communication system (such as a PDC (Personal Digital Cellular), GSM (Global System for Mobile communications, and so forth) for rendering communication service adopts a TDMA (Time Division Multiple Access) form in which a time slot is assigned for each user, and thereby communication is rendered. This form was designed mainly for providing voice communication service, and renders voice communication service rendering transmission of voice information at a fixed rate.
Further, with regard to application of a multi-carrier CDMA radio transmitting system to a digital mobile communication system, studies have been made recently. In the studies, discussion has been made mainly for accommodating more users (mobile stations) on condition that information is transmitted at a same transmission rate.
When considering transmission of multimedia information including image information (static images/pictures, dynamic images/pictures) and voice information, it is preferable to made the transmission rate of information variable depending on types of information to be transmitted, state of channel between a base station and a mobile station, information processing capability of a reception-side apparatus, and so forth.
The multi-carrier CDMA radio transmitting system employs a spread spectrum communication method in which an information symbol is multiplied by a spreading code for each user on a frequency axis. An analysis for information transmission at different transmission rates for the multi-carrier CDMA radio transmitting system rendering information transmission for each user by such a method has been made. However, no concrete method for this purpose has not been reported clearly.
In a mobile communication environment, amplitude variation and/or phase variation in a communication channel due to Rayleigh fading caused due to change in relative positional relationship between a mobile station and a base station occur. Therefore, it is necessary to identify a phase of a received signal by an absolute phase for each information symbol in a phase modulation form in which information is transmitted at a carrier phase.
For the above-described requirement, ‘Pilot Signal Inserting Form for Multi-Carrier Modulating Form employing 16QAM (Yamashita, Hara, Morinaga: Spring Meeting of the Institute of Electronics, Information and Communication Engineers, B-256, pages 2-356, March, 1994)’ and ‘Discussion of Adapting Form for OFDM by using Pilot Symbol (Yamashita, Kuwabara, Itami, Itoh: General Meeting of the Institute of Electronics, Information and Communication Engineers, B-5-245, page 609, September, 1998)’ propose a method of estimating and compensating fading distortion by using pilot symbols, the phases of which are known, inserted among appropriate plurality ones of all the sub-carriers and also among information symbols at fixed periods.
By this method, for example, as shown in FIG. 1, by using pilot symbols (●) inserted into a plurality of sub-carriers f1, f2, . . . at fixed periods, amplitudes and phases of received signals for respective users are measured, and the measured values are obtained through interpolation rendered two-dimensionally in the time-axis direction and sub-carrier direction (frequency direction), and, thereby, variations in transmission channel for information symbols are estimated. Then, based on this estimation result, phase rotations of data symbols are compensated, and coherent detection is rendered. In this method, in order to reduce power consumption due to insertion of pilot symbols, the technique of interpolation is used without inserting pilot symbols into all the sub-carriers on the supposition that correlation in channel variation between respective sub-carriers is very high, and variations in transmission channel for sub-carriers into which no pilot symbol is inserted are estimated (channel estimation).
However, when the information transmission rate is increased, and the occupied frequency band is widened, the correlation in variation in transmission channel between respective sub-carriers vary depending on delay amounts due to the influence of delayed wave (echo). Thereby, the correlation in variation between sub-carriers may be decreased. In such a case, it is not possible to estimate channel variation precisely by using a pilot symbol of a sub-carrier apart by several carries.